1. Field of the Invention
This invention relates to Permanent Magnetic Devices. More particularly, this invention relates to structures for producing magnetic flux paths with permeability greater or lesser than that of air (1.0), to magnetic circuit design, to attaining variable magnetic reluctivity and the adjustment of a magnetic field's permeance by both mechanical and temperature control means.
2. Description of the Prior Art
There are a number of technological fields in which it is desirable to have magnetic circuits designed which fix or vary the amount of reluctivity or permeability in such circuits, and consequently control the leakage of magnetic flux. It is well defined in the magnetic arts that permeance and reluctivity have an inverse relationship based on the formula P=I/R. Reluctance is defined as the opposition that material offers to magnetic lines of force, while permeance is defined as the ease with which magnetic lines of force distribute themselves within a material.
The current demand for compact, strong, static magnetic field sources that require no electric power supplies has created needs for permanent magnet structures of unusual form. There has been increasing interest in applications using permanent magnet structures. A number of configurations have been designed and developed for electron beam guidance in mm/microwave tubes of various types: for dc biasing fields in millimeter wave filters, circulators, isolators, strip-lines, for field sources in nuclear magnetic resonance imagers, extended interaction amplifiers, klystrons, travelling wave tubes, magnetrons and so on.
Many devices that employ magnetic fields have heretofore been designed where magnetic flux leakage is reduced or eliminated by employing one or more cladding magnets to confine the magnetic flux to the working space. Similarly, longitudinal magnets have been combined or aligned in such a way that the leakage of magnetic flux is reduced or eliminated. See for example, Leupold, et. al. "A Catalogue of Novel Permanent-Magnet Field Sources" Paper No. W3.2 at the 9th International Workshop on Rare-Earth Magnets and Their Applications, Bad Soden, Federal Republic of Germany, Aug. 31-Sep. 2, 1987. Further, adjustable magnetic fields have a great number of practical uses and have been known for some time in the permanent magnet arts. See for example Leupold, U.S. Pat. No. 4,837,542, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source for Producing a Uniform High Field"; Leupold, U.S. Pat. No. 4,862,128 entitled "Field Adjustable Transverse Flux Sources"; Leupold, U.S. Pat. No. 4,994,778 entitled "Adjustable Twister"; Leupold, U.S. Pat. No. 5,113,163 entitled "Adjustable Magnetic Field Superconducting Solenoid" and Leupold, U.S. Pat. No. 5,216,400, entitled "Magnetic Field Sources for Producing High-Intensity Variable Fields."
Those concerned with the development of such systems have long recognized the need for magnetic structures capable of controlling the delivery of magnetic flux from a magnetic flux source to a load.
Unlike electrical circuits, magnetic flux flow paths are not confined to tractable paths such as electrical wires. Magnetic flux lines tend to fill all space in their proximity, however it is possible to bunch or manipulate a magnetic flux path by utilizing iron magnetic circuit elements which have zero reluctance. In magnetic circuits, the greatest reluctivity is generally that of air wherein permeability, or .mu., equals 1.0. Ordinarily, flux paths have permeabilities of either .infin., infinity, as in the case of iron, or 1.0, in the case of air or rigid permanent magnets, with occasional values between 1.0 and .infin., in the case of non-rigid permanent magnets and paramagnets. Between air and iron are a number of paramagnets with a permeability between 1.0 and .infin., and these paramagnets can act as intermediate reluctors that will increase the magnetic circuit's permeability to values higher than that of air. Inserting such a reluctor results in the surrounding airspace furnishing a magnetic flux path in parallel with the inserted object. For permeability values between 1.0 and 0, the medium must be diamagnetic. The only known diamagnets of appreciable strength are the superconductors, and for values of permeability between 0 and 1.0, only some of the Type II superconductors perform adequately.
Thus, choice of materials and their alignment or manipulation within the magnetic circuit allows adjustment of the total permeance of the magnetic circuit. The first reluctor structure disclosed herein produces a more or less constant reluctance for the given structure by alignment of a plurality of baffled reluctor members within the structure's magnetic flux flow path. Often, however, applications require that the magnetic field be adjusted through a range of values making it desirable to create magnetic fields that can be readily adjusted with continuous variation of permeance from 0 to 1.0 (air). The present invention fulfills this need by also providing a reluctor member that can be placed within a structure's flux path so that permeability can be readily adjusted, either by alignment of the reluctor member or insertion by mechanical means. Providing a permanent magnetic structure of this type allows fixing or varying the permeability of the flux path and thus the characteristics of the flux being produced. The reluctor member can comprise stacked pieces, or disks, constructed of iron, a superconducting material, or a combination of iron, superconductors and magnetic material. Additionally, this invention provides for attaining variable permeance by employing thermal controls with reluctor member pieces, or disks, composed of either concentric annular rings or parallel strips constructed of superconducting elements with different transition temperatures. The present invention also provides for methods of adjusting or varying magnetic permeability.